The Old Bailey heard how Smith, 20, who suffers from Asperger's Syndrome, built the device at home after googling an al Qaeda article entitled Make A Bomb In The Kitchen of Your Mom.

On Wednesday his mother Antonitza Smith, 47, told the Telegraph she blamed YouTube for influencing her son and called for it to be banned.

Passengers were ordered off a train at North Greenwich Station Credit:
AFP/Getty Images

"How to make a bomb - that shouldn't be on YouTube because people copy, especially vulnerable people," she said.

"Whatever people put on YouTube, broadcast on YouTube - how to make a bomb, how to blow up a car, hacking etc - it's illegal really. They talk about terrorism, beheadings. That shouldn't be on YouTube so young kids could see it. It's wrong.

"The government should ban it so it might save another child from what my son has gone through."

Smith admitted being interested in Islam but denied being an extremist even though he posed next to an image of the Brussels-born Islamic terrorist alleged to have masterminded the attacks in Paris in November 2015.

He was not charged with terror offences because there was no evidence that his actions were for the purpose of a political, religious or ideological cause, according to prosecutors.

He told police he had been inspired after watching someone on a YouTube channel called Trollstation doing a bomb prank and had only intended to release smoke from the device.

Asked what sparked his interest in bomb making and extremism, his mother said: "It all started off, he got interested in the Quran when he was taking his GCSEs. So my mum bought a Quran for Christmas for him and he got interested in all the predictions. That was what he was interested in mostly.

North Greenwich tube station was at the centre of the security alertCredit:
Philip Toscano/PA Wire

"I don't know why he was interested in bombs. I suppose it was being a boy and his Asperger's as well.

"Maybe a couple of years ago I noticed. He was showing me how to make bombs but I knew he was safe at home."

A search of the home in Rotherhithe, south London, that he shared with his mother, later revealed his fixation with guns, explosives and other weapons.

His mother said her son was bullied and turned to his computer.

"As Damon has got Aspergers, he has got access to weird sorts of topics on the Internet," she added.

"Because people with learning disabilities, if they get these funny ideas, they start copying. It can be dangerous especially if you have got autism.

"He is like a little baby. He can't talk properly.

"I think YouTube is poison. They talk about terrorism, beheadings. That shouldn't be on YouTube so young kids should see it. It's wrong."

YouTube says it removes such footage as soon as they are alerted to it.

“We do not allow bomb-making videos on YouTube and if there’s content giving instruction on how to carry out harmful activities on the platform, we quickly remove it when it’s flagged," a spokesperson said.

Police seized a blank-firing self-loading pistol and a BB gun from his home, both bought legally, as well as a knuckle duster and a knife, which he showed off in an online video.

A jury at the Old Bailey took two hours to find Smith, 20, guilty of possession of an explosive substance with intent.

He had already admitted the lesser offence of making a bomb hoax.

Smith, who smiled throughout his trial, made no reaction in the dock as the verdict was given.

Ms Smith said she was not in court for the verdict, as she found it "too difficult", but was "shocked" when she heard and maintains her son is "not an extremist".

Since his arrest he has been held in maximum security HMP Belmarsh, where, according to his lawyers, he has been the target of attacks by other inmates because of his high-pitched voice.

Damon Smith smiled throughout his trial at the Old BaileyCredit:
Elizabeth Cook/PA

The jury had heard how on the morning of October 20 last year, Smith, then aged 19, packed a rucksack with explosives and deadly ball-bearing shrapnel as he headed off to college in Holloway, north London.

He was caught on CCTV as he travelled on the Jubilee Line, casually flicking through a text book before getting off and leaving the bomb on the floor, timed to go off within minutes.

At least 10 passengers were in the carriage at the time and some of them spotted the abandoned rucksack and alerted the staff.

The driver initially thought it was lost property and took it into his cab before eventually realising the danger and raising the alarm.

The Old Bailey heard that had Smith's bomb worked, it would have exploded just as commuters were being ordered off the platform.

Smith went on to college and, on returning home in the evening, checked the internet for news of what he had done.

Upon his arrest by counter-terrorism officers, he admitted making the bomb, but insisted it was only meant to spew out smoke as a Halloween joke.

Smith, who grew up living with his mother in Newton Abbot, Devon, said he had thought about putting a bomb in a park but decided it would be "more funny" to delay train passengers.

His lawyer, Richard Carey-Hughes QC, told jurors that Smith was no "hate-filled jihadi", but a “white boy from Devon” who was “tied to his mother’s apron strings”.

Judge Richard Marks QC adjourned sentencing until May 26 to allow time for a probation report on Smith's risk to the public and further psychiatric reports.